(1) Field of the Invention
The present invention relates to a copper foil for printed circuits, and more particularly to surface treatment on the shiny side thereof, and further particularly to a method of surface treatment for obtaining a copper foil for printed circuits having a shiny side, hardly allowing, when pressing with a resin substrate for printed circuits, particles of the applied resin substrate to fuse on the copper foil shiny side, and possessing heat discoloration resistance, solderability, resist adhesion, and rusk preventiveness.
(2) Description of the Prior Art
Printed circuit boards are rapidly advanced every year, and requirements for higher performance and higher reliability are mounting. Accordingly, they are becoming more and more complicated and diversified. As conductive materials for printed circuits, copper foils are widely used, and strict quality demands are similarly imposed on these copper foils.
A first step in manufacture of a printed circuit board is to laminate a copper foil and a synthetic resin impregnated material, with the rough side of the copper foil abutting against the synthetic resin impregnated material, and to heat and press by a press machine to obtain a copper clad laminate plate. As the copper foil for printed circuits, an electrolytic copper foil is mainly used, and generally the non-shiny side is toughened in the electrolytic copper foil, and either side is roughened in the rolled copper foil, in order to increase adhesion with the resin, and various techniques have been developed for stabilizing the heat resistance, chemical resistance, adhesion characteristic, etching characteristic and others in order to assure adhesion with the resin, and thereby a high functional surface is provided.
On the other hand, completely different properties are demanded on the opposite side, that is, on the shiny side, such as heat discoloration resistance, solderability, and resist adhesion, and therefore different treating methods are needed and studied for the rough side and shiny side. The copper foils for printed circuits have been established as products requiring such advanced and characteristic treating technologies.
In the glass epoxy substrate that is used widely, a heating and pressing process is conducted for 1 or 2 hours at 160.degree. to 170.degree.C., but special high heat resistant resins such as glass polyimide substrates that are becoming popular recently need heating at 200.degree. C. for 1 or 2 hours, or even at higher temperature and for a longer time.
Moreover, higher heat resistance is more often demanded year after year, such as post-curing for a long time at high temperature after a laminating process in order to improve dimensional precision. In other materials than glass epoxy substrates, such as flexible printed wiring boards, too, heat treatment at high temperature for a long time is being done. Accordingly, of the shiny side characteristics of the copper foil, the heat discoloration resistance is one of the particularly important characteristics.
Yet, when resin dust generated from the resin substrate mixes onto the copper shiny side at the time of lamination, the resist by the resin is formed in the area after the heating and pressing process, and this area is not removed by etching, and the copper is left over to cause a fatal defect such as shorting of circuits, and the value of the copper foil laminates is completely lost, which has been recently disclosed as a problem.
In the industry, various improvements have been attempted, such as cleaning of the field of heating and pressing process works and prevention of drops of prepreg dust, but if the resin dust deposits on the copper foil shiny side, the copper foil is required to have such surface treatment that it may not adhere and fuse on the copper foil at the time of heating and pressing. Hereinafter, the preventive effect of such resin dust adhesion is called resin dust resistance. This problem is particularly caused by the epoxy resin dust in FR-4, G-10, and CEM-3, and at the present its improving method has not been technically developed yet.
In the prior art, various techniques for offering heat discoloration resistance have been proposed, and, for example, Japanese patent publication No. 54-29187 discloses a method of immersing copper in an alkaline aqueous solution containing zinc, or being electrolized in the aqueous solution, using the copper as the anode, and Japanese patent publication No. 58-7077 discloses a method of forming a coating layer of a mixture of zinc or zinc oxide and chromium oxide. Besides, Japanese patent laid-open publication No. 5-140765 discloses a method of chromate treatment after forming a Zn-Ni alloy layer.
These conventional methods commonly have the defect of adhesion of the resin dust generated from the resin substrate, especially epoxy resin dust, to the copper foil shiny side at the time of lamination, thereby fusing and forming the resin dust in the area by heating and pressing. That is, all methods are extremely poor in the resin dust resistance. On the other hand, a layer of zinc or a layer of zinc or zinc oxide and chromium oxide or the like is excellent in solderability, but inferior in heat discoloration resistance and rust preventiveness, or a method of forming a chromate treated layer after forming a Zn-Ni alloy layer has a problem in solderability. Besides, chromium oxide or chromate treated layer is formed by using hexavalent chromium, but its use is not preferred from the viewpoint of environmental problem of which regulation is becoming stricter nowadays.